docs: def get_graph(self, name):
          """Return a graph previously created with ``new_graph``,
          ``new_digraph``, ``new_multigraph``, or
          ``new_multidigraph``
  
          :arg name: name of an existing graph
  
          """
          if name in self._graph_objs:
              return self._graph_objs[name]
          graphtypes = {
              'Graph': Graph,
              'DiGraph': DiGraph,
              'MultiGraph': MultiGraph,
              'MultiDiGraph': MultiDiGraph
          }
          type_s = self.query.graph_type(name)
          if type_s not in graphtypes:
              raise GraphNameError(
                  "I don't know of a graph named {}".format(name)
              )
          g = graphtypes[type_s](self, name)
          self._graph_objs[name] = g
          return g